In mobile communications as typified by a mobile phone or the like, low costs, high integration, low power, multifunction, speed improvement, and others evolve day by day. So-called RF (radio frequency) devices are used in these aspects. A compound semiconductor is exclusively used for such RF devices.
However, in recent years, since a CMOS process has been finer or a reduction in cost is desired, an RF device based on silicon has become feasible.
In an RF device using a silicon single crystal wafer, when a substrate resistivity is low, loss is high because of high conductivity, and a high resistivity is used. Although a wafer having a thin oxide film and a thin silicon layer formed on a silicon substrate surface layer portion, called an SOI (Silicon on Insulator), may be used, a high resistivity is likewise desired in this case.
However, when a high-resistivity substrate of crystal grown by the CZ method is used, there occurs a problem that oxygen atoms that are present in CZ crystal form oxygen donors by a heat treatment at a relative low temperature for wiring or packaging which is a post-process of a device and thereby a resistivity of the substrate shifts.
In Patent Literature 1 or Patent Literature 2, there is suggested a method for outwardly diffusing oxygen atoms by performing a high-temperature heat treatment, thereby lowering concentration near the surface of the substrate where a device is formed.
However, according to this method, there is a problem that a high-temperature heat treatment process is required and hence costs are increased.
Thus, lowering oxygen concentration in crystal that is grown by the CZ method is considered to be preferable. Patent Literature 3 discloses an oxygen reducing technology, and considerably low oxygen concentration is achieved. However, as described in Patent Literature 3 or the like, a top side (a head side) of crystal generally has high oxygen concentration, lowering this concentration is difficult, and hence an good chip yield is decreased as the target oxygen concentration that should be achieved becomes lower.
Furthermore, when the target oxygen concentration is extremely low, material melt in the crucible starts to be solidified and dislocation of crystal occurs in some cases. Therefore, if the considerably low oxygen concentration is aimed, there occurs a problem that costs increase in the end.
Patent Literature 4 discloses a method for obtaining an amount of oxygen donors generated in high-resistivity crystal. Here, it is assumed that the amount of the generated oxygen donors is proportionate to the Bth power of the oxygen concentration (see paragraph 40 in Patent Literature 4), but a specific numerical value is not eventually revealed.
Moreover, FIG. 2 of Patent Literature 4 shows that the oxygen concentration and the amount of the generated oxygen donors have a relationship that is expressed by a straight line on a semilogarithmic graph. If the amount of the generated oxygen donors is proportionate to the Bth power of the oxygen concentration, the straight line relationship on the semilogarithmic graph cannot be obtained, the amount of the generated oxygen donors is not represented as a mathematical expression by this method after all, and hence it can be considered that general versatility is not provided.